1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer, a facsimile, or a copying machine.
2. Description of the Related Art
In an image forming apparatus, there is a demand for stably obtaining high-quality images for a long period of time, but time-dependent deterioration in toner may cause changes in image quality. For example, in an image forming apparatus which uses two-component developer containing toner and carriers to form toner images, toner deterioration occurs due to repetitive stress caused by a developer regulating member or the like which is provided to optimize the amount of developer on the developer carrier, causing changes in the charged toner amount. Thus, desired image density may not be obtained, or background fouling, interior contamination due to toner scattering or the like may occur.
Similarly to two-component developer, in one-component developer, toner deterioration occurs due to repetitive stress caused by a developing roller, a developer regulating member, and the like, and additives attached to the surfaces of toner particles are buried in the toner particles or separated from the toner particles. The occurrence of such an event causes a significant difference in toner charge amount between deteriorated toner and newly replenished toner, resulting in background fouling or erraticness.
In an intermediate transfer type image forming apparatus, deterioration of developer makes the charge amount unstable, resulting in unstable transfer of a toner image from an image carrier to an intermediate transfer element. For this reason, primary transfer efficiency is degraded, and deterioration of an image transferred onto a transfer material, such as paper, tends to be noticeable.
As described above, if toner deterioration occurs and developer is deteriorated, image quality of an image transferred onto a transfer material, such as paper, is degraded. The major factor of the above-described phenomenon is deterioration of toner in developer. Thus, deteriorated toner is discharged, and new toner is put in developer and stirred, thereby obtaining a good-quality image on a transfer material, such as paper.
In the relate art, a technique is known in which, when it is considered that the ratio of deteriorated toner increases, toner in developer is replaced so as to obtain high-quality images over a long period of time. For example, Japanese Patent Application Laid-open No. 2006-171788 describes an apparatus in which, when an image area is small and toner consumption is low, toner is forcibly consumed in a region other than an image region. When toner consumption is low, toner undergoes repetitive stress and toner deterioration is noticeable. Thus, toner is forcibly consumed in a region other than an image region, toner in developer is replaced, and then, development is carried out. However, according to this method, wasteful toner consumption occurs, leading to an increase in cost. Further, in this method, instead of detecting the degree of toner deterioration and replacing a necessary amount of toner, if it is anticipated that toner consumption is low, toner is replaced. For this reason, it is unclear how much it is possible to cope with actual toner deterioration. As described above, the technique of the related art in which deteriorated toner in developer is replaced has various problems. Thus, there is a demand for obtaining high-quality images even when deteriorated toner is contained in developer.
A technique is known in which an actual toner adhesion amount is measured, and process control is performed so as to obtain high-quality images over a long period of time. For example, Japanese Patent Application Laid-open No. 2007-101980 describes a technique in which a toner pattern formed on a photosensitive element is transferred to an intermediate transfer element, and the toner adhesion amount of the toner pattern on the intermediate transfer element is detected and fed back to the toner image forming conditions, such as the charging condition and the developing condition, on the photosensitive element. According to this technique, an actual toner adhesion amount is measured and the toner image forming conditions are determined, making it easy to obtain an appropriate toner adhesion amount.
However, even when an appropriate toner adhesion amount on an image carrier is obtained by the above-described process control, if toner which is deteriorated with time is included in a toner image formed on the image carrier, deteriorated toner tends not to be easily transferred, and the toner adhesion amount of the toner image formed on the recording medium is not uniform, leading to degradation in image quality.
As the index of image quality, smoothness of gradation is exemplified. The gradation is a change in color density, that is, the steps of color. For example, in the case of white and black, there is gray as an intermediate color, and gray includes light gray and dark gray. If many steps are set, in the case of two colors of white and black, a picturesque image with smooth color changes can be formed. From among the gradations, the brightest gradation portion is called highlight, an intermediate gradation portion is called halftone, and the darkest gradation portion is called shadow. Of these, the halftone portion is largely affected by deteriorated toner. With the effect of deteriorated toner, degradation in image density is noticeable in the halftone portion, causing degradation smoothness of gradation. FIG. 15 shows a relationship between an input image area ratio and image density for new developer and deteriorated developer. As shown in FIG. 15, when deteriorated developer containing deteriorated toner is used, image density in the halftone is degraded compared to new developer, and a change in image density is not uniform, making it difficult to obtain appropriate density gradation.